1. Technical Field
The invention disclosed broadly relates to the switching of multiphase electrical power sources and more particularly relates to the precise identification of zero crossover for the relative phases in a polyphase system.
2. Background Art
Delta power systems and some unreferenced wye power systems are commonly used in power generation and distribution networks. These are three phase voltage systems, the waveform of which is shown in FIG. 1. Conventionally, three phase voltage waveforms are represented by phase A, phase B and phase C which are generated so as to be 120 degrees in phase separation, respectively. The measurement of the relative timing is generated at the respective phases crossing zero volts is important in power switching control applications. For example, reference is made to the U.S. Pat. No. 4,761,563 to Ross and Woodworth, entitled "Asynchronous Multiphase Switching Gear," wherein FIG. 28 shows a waveform coincidence detector circuit which requires the detection of the zero crossover for each respective phase A, B and C, in order to carry out the operation of synchronously transferring three phase power from a first power source to a second power source. This is just one example of many which could be provided of the need to identify when the phases cross zero volts. Proper phasing and control of electronic switching devices often rely on this timing information. The actual passage of a phase through zero cannot be recorded accurately at times due to power line noise. The displacement of the imaginary neutral voltage reference point from ground due to unbalanced ground currents and system switching delays cause significant errors. This can be better understood with reference to the vectorial representation of a three phase power system, as is shown in FIG. 2. FIG. 2 illustrates a vectorial representation of the three phases A, B and C as an equilateral triangle. It is seen in the six views of the vectorial triangle in FIG. 2, that the triangle appears to be rotating in a clockwise direction. This represents each of a 60 degree increment in the rotation of the three phase generator supplying the power to the three phase system. The geometry of the three phase waveforms as shown in FIG. 2, can be described as follows. In a three phase alternating current (AC) system, one phase, for example phase A, will cross through an imaginary zero voltage point when the other two phase voltages, for example B and C phases, are equal and opposite in magnitude. This can be seen in the diagram of FIG. 2 which shows the vectorial representation of a three phase power system and occurs twice per phase in a full revolution of the three phase generator. The crossings occur at both the negative to positive and at the positive to negative polarity crossings. FIG. 2 has six views labeled 1 through 6. In view 1, a positive going crossing is illustrated for the A phase. In view 2, a negative going crossing is illustrated for the B phase. In view 3, a positive going crossing is illustrated for the C phase. In view 4, a negative going crossing is illustrated for the A phase. In view 5, a positive going crossing is illustrated for the B phase. In view 6, a negative going crossing is illustrated for the C phase. The prior art has not provided a reliable, noise tolerant technique for identifying the zero crossing of the respective phases of a polyphase power system.